1. Field of the Invention
This invention relates to a silicon carbide semiconductor device and more particularly to a silicon carbide semiconductor device having an aluminum nitride single-crystal layer as an electrically insulating layer.
2. Description of the Prior Art
Silicon carbide (SiC) is a semiconductor material with a wide band gap of 2.3 to 3.3 electronvolts (eV), which is thermally, chemically and mechanically quite stable, and also has a great resistance to radiation damage. Furthermore, the saturation drift velocity of electrons in silicon carbide is greater than that in silicon (Si) and other semiconductor materials. The use of semiconductor devices made of conventional semiconductor materials such as silicon is difficult under severe conditions of high temperature, high output drive, high frequency operation, and radiation exposure. Therefore, semiconductor devices using silicon carbide are expected to have wide applications for devices which can be used under such conditions without degrading their characteristics.
Despite these many advantages and capabilities, a silicon carbide semiconductor device has not yet been put into actual use, because a technique still remains to be established for growing high quality silicon carbide single crystals having a large surface area with good reproducibility which is required for the commercial production thereof.
In recent years, there has been developed a process for growing large-sized high-quality single crystals of silicon carbide on a single-crystal substrate of silicon by the chemical vapor deposition (CVD) technique (e.g., Japanese Laid-Open
Patent Publication No. 59-203799). This technique makes it possible to control the conductivity type, the impurity concentration, or the like of silicon carbide single crystals obtained by adding an appropriate amount of impurities during the growth of the single crystals. Therefore, this technique makes many contributions to the development of various semiconductor devices in which silicon carbide single crystals are used.
In conventional silicon carbide semiconductor devices, a device active region is formed in a silicon carbide single-crystal layer grown directly on a silicon single-crystal substrate.
However, the lattice constant of silicon single crystals is different from that of silicon carbide single crystals by as much as about 20%, so that there are many crystal defects due to the lattice mismatching (e.g., stacking faults) within the silicon carbide single-crystal layer grown on the silicon single-crystal substrate. These crystal defects cause the generation of leakage current in the silicon carbide semiconductor device. The leakage current flows in the silicon carbide single-crystal layer, the interface between the silicon single-crystal substrate and the silicon carbide single-crystal layer, and the silicon single-crystal substrate via the crystal defects. The generation of leakage current degrades the characteristics of semiconductor devices.
Therefore, silicon carbide semiconductor devices with the above-mentioned structure, which have been developed recently, do not have satisfactory device characteristics and cannot be used for practical applications.